Some of the kids at Stoneybrook Elementary School have decided to hold a "Sitter of the Month" contest and all the club members hope to win, especially Dawn, who thinks she has good shot of being the very first sitter of the month.

When Dawn babysits for the Prezziosos, the phone rings, but nobody's there & then the doorbell rings, but nobody's at the door, but she does find a note from Mr. X. Dawn doesn't tell anyone about the note because she'll think it'll ruin her chances of winning "Sitter of the Month." The same thing happens when she's sitting for the Rodowskys.

Dawn isn't the only BSC member to get visits from Mr. X: Jessi gets a bouquet of dead flowers with an note in them while she's sitting at home; Mary Anne & Mallory get a note threatening the Pikes' hamster, Frodo. Everyone in the club gets notes from Mr. X except for Kristy, who has been acting very competitive. Dawn believes that Kristy is "Mr. X." in order to ruin everyone else's chances of being the first "Sitter of the Month."

The girls decide to put their differences aside and work together to help find who is really "Mr. X," but they don't have any clues or suspects. When Dawn babysits for Jamie & Lucy Newton, Jamie tells her that Mel Tucker will be stopping by to do a secret baby-sitting check. Dawn lets the other girls know about what Jamie told her and they decide to set a trap for Mr. X.

Dawn tells the other club members that she'll be sitting for a fictional cousin of hers at her house one night. The rest of the BSC sneaks over to Dawn & Mary Anne's house and as soon as they hear suspicious noises in the secret passage, Dawn opens the entrance and discover Mel Parker. Mel takes off running, hoping to escape through the other end of the passage, but the rest of the BSC is waiting in the barn for him.

When Kristy and Dawn walk Mel home, Mel confesses to being Mr. X. He was mad at the BSC after Dawn let Mrs. Hobart know that Mel & his friend, Zach were still calling the Hobart boys "Croc." Mrs. Hobart called Mel's parents and Mel got grounded. He discovered where everyone would be sitting by letting the other kids at SES think that it would help with the "Sitter of the Month" contest. The reason Kristy wasn't targeted because she had been sitting in her own neighborhood lately which would make it tough for Mel to sneak out of the house and go all the way to Kristy's neighborhood.

Kristy & Dawn talk to Mel's parents, who decide to send him to a child psychologist for help. In the end, the "Sitter of the Month" contest ended in a 7-way tie.